From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys and intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, B. Batlogg, Physica, Vol. 126, pp. 275-279 (1984), which reviews superconductivity in barium bismuth lead oxide, and J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B-Condensed Matter, Vol. 64, pp. 189-193 (1986), which reports superconductivity in lanthanum barium copper oxide.
The latter report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y-BA-Cu-system can have superconductive transition temperatures T.sub.c above 77K, the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58(9), Mar. 2, 1987, pp. 908-910; and P. H. Hor et al, ibid, pp. 911-912). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of compositions and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90K (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16) pp. 1676-1679 (1987)).
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be perhaps one of the most advantageous cryogenic refrigerant, and attainment of superconductivity at liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials.
The prior art knows several techniques for producing superconductive oxide bodies, including superconductive thin films. See, for instance, U.S. patent application Ser. No. 089,296, "Robust Superconductors", filed Aug. 25, 1987 for R. E. Howard et al now abandoned; U.S. patent application Ser. No. 126,448, "Method of Producing a Layer of Superconductive Oxide, and Apparatus and Systems Comprising a Layer Produced by the Method", filed Nov. 30, 1987 for M. E. Gross et al; and R. B. Laibowitz et al, Physical Review B, Vol. 35(16) pp. 8821-8823 (1987). Superconductive oxide films have been deposited, for instance, by laser and electron beam evaporation, sputtering, co-precipitation, or plasma-arc spraying.
A significant portion of the research on high T.sub.c oxide superconductors is directed towards increasing the critical current density J.sub.c. This is the maximum supercurrent density a body of the superconductor material can support, at a given temperature and in the absence of an applied magnetic field. Substantial progress has already been made in this respect, with J.sub.c &gt;3.times.10.sup.6 A/cm.sup.2 at 77K reportedly having been observed in some thin films of a variant of the prototypical "1-2-3" compound YBa.sub.2 Cu.sub.3 O.sub.7. It will be appreciated that for most technological applications large J.sub.c is of the essence.
Even though in general it is desirable for a superconductor to have the highest possible J.sub.c, in at least some applications this is not the case. For instance, the critical current and geometry of a SQUID (superconducting quantum-interference device) advantageously are chosen such that 2I.sub.o L/.phi..sub.o .apprxeq.1, where I.sub.o is the critical current, L is the SQUID self-inductance, and .phi..sub.o is the fundamental flux quantum. See, for instance, R. H. Koch et al, Applied Physics Letters, Vol. 51(3), pp. 200-202 (1987). Since the value of .phi..sub.o is a constant and L frequently can not easily be reduced beyond some value in the order of 10.sup.-9 H, the above condition can frequently only be met if I.sub.o is relatively low. This in turn may require that J.sub.c be relatively low.
In view of the need to have available superconductive films having a relatively low predetermined value of J.sub.c, a simple, convenient method for reducing the J.sub.c of all or part of a thin film superconductor would be of significant interest. This application discloses such a method.